Despite significant developments in fiber laser technology in recent years, there are still great needs to scale power in both continuous wave and pulsed lasers for use in a wide range of industrial, scientific and defense applications. Optical nonlinear effects, such as stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), self-phase modulation (SPM) and Four-wave-mixing (FWM) are some of the key limiting factors in power scaling. All of these nonlinear effects could be mitigated by effective mode-area scaling of fibers while maintaining single-transverse-mode operation. Effective mode-area scaling could also lead to high pulse energy due to an increase in stored energy in the amplification process.
Although numerous techniques have been studied for the suppression of the wide range of nonlinearities, the fundamental solution to power scaling is scaling of mode areas in fibers while maintaining sufficient single mode operation. The key problem to be overcome is that fundamental physics states that more modes are supported once the physical dimensions of waveguides are increased.
Approaches that have been studied to solve this problem generally fall into three categories. The first involves reducing the numerical aperture of the waveguide. Since the number of modes supported by a waveguide is a function of both core diameter and numerical aperture, a lower numerical aperture can be used to reduce the number of guided modes. Some early approaches of mode-area scaling, photonic crystal fibers and recent triple clad approaches fall into this category. One major deficiency of these approaches is that a lower numerical aperture weakens the fundamental mode guidance and renders it very sensitive to bending and any other mechanical perturbation on the fibers. Thus, photonic crystal fibers with lower numerical apertures and over 40 μm core diameter can only be used as straight rods.
The second category includes approaches based on the introduction of differential mode losses. Here, fundamental mode guidance is strong enough to allow coiling even at large core diameters while higher order modes are eliminated by introducing higher losses for these modes. Conventional step index large mode area fibers fall into this category and differential mode loss in this case is from coiling. This approach exploits strong mode-dependent loss to mitigate the waveguide's tendency to support more modes at large core diameters. One major benefit of this approach is that strong fundamental mode guidance can be maintained to allow coiling. The major challenge is to introduce very high losses for all higher order modes at the desired wavelength while maintaining good fundamental mode transmission.
Special waveguide designs have also been developed to further increase differential mode loss. Some recent approaches rely on resonant out-coupling of higher order modes from a conventional step-index core. A low loss is ensured for the desired fundamental modes by the conventional cores. These systems are, however, limited in terms of scaling much beyond 50 μm core diameter. The higher order mode out-coupling fundamentally relies on phase-matching, typically at a different wavelength for a different mode, and spatial overlap between the modes. However, both of these aspects become major limits very quickly in a large core fiber. As such, it becomes difficult to ensure that all phase-matching conditions are met at the same desired wavelength for all relevant higher order modes when there are a number of modes in consideration. As the core diameter increases, these higher modes in the core are increasingly more confined to the core center. This leads to much less mode coupling due to a reduced spatial overlap between the coupling modes and much stronger wavelength-dependence in phase-matching. Both make these designs hard to implement, especially at large core diameters. Other approaches in this second category include leakage channel fibers. They overcome the limitations of the resonantly-coupled approach by starting with a leaky waveguide. Because modes are no longer guided in a leaky waveguide, a significant new way for optimizing differential mode loss is possible. Since these designs do not necessarily depend on any resonant effects, they are much more tolerant in the fabrication process. Due to the delocalized nature of modes, they are more scalable to much larger core diameters. Single mode operation in a core diameter of 180 μm has been demonstrated in leakage channel fibers.
The third category of approaches for mode area scaling is based on the operation of one of the higher order modes (HOM) in a highly multimode fiber. This approach works upon the premise that the propagation of a higher order mode can be very stable even in a highly multimode fiber. Moreover, these higher order modes can offer significantly better bending performance. The main deficiency of this approach is that in an active highly multimode fiber, spontaneous emission populates all modes equally by fundamental quantum mechanical principles. This can significantly limit the operation of high gain amplifiers due to strong amplified spontaneous emission (ASE) in undesired modes. While complex techniques have been proposed recently to mitigate these limits, it is hard to completely eliminate this ASE problem. Mode area scaling to 20 μm mode field diameter using all-solid photonic bandgap fibers has been reported. A detailed theoretical investigation on the limit of mode area scaling with all-solid photonic bandgap fibers indicated an upper limit of about 500 μm2 using a more optimized seven-cell core and operating in the first bandgap. Recently, all-solid photonic bandgap fibers with up to about 700 μm2 effective mode areas have been demonstrated operating in the first bandgap.
What are needed in the art are optical fibers that can provide high power output, such as may be used in high power lasers, and methods for forming the high power optical fibers.